This invention relates to microelectromechanical mirrors and mirror arrays, and a method for manufacturing the same.
As the internet has grown, so too has the strain on the telecommunications infrastructure. As more and more information is transmitted across the Internet, and the demand for information rich content like streaming video has grown, telecommunication providers have struggled to provide the necessary bandwidths and data rates necessary to carry the requisite data. To that end, telecommunications providers have looked to carrying more and more data on fiber optic networks, and to extending the reach of their fiber optic networks beyond the network backbone.
One limitation of fiber optic networks as currently implemented is their inability to directly switch optically encoded packets of data from a fiber on a source network or network node to a fiber on a destination network or network node. Instead, the optically encoded data is dropped from the source network fiber, converted to electrically encoded data, switched to the destination network using conventional electronic switches, converted back into optically encoded data, and injected into the destination network fiber.
Micromachined mirror arrays offer the ability to directly switch optically encoded data in devices, known as all-optical cross connect switches, from a source fiber on a source network to a destination fiber on a destination network without having to convert the data from optical to electronic and back again. For such mirror arrays to be commercially useful, they must be able to cross connect approximately 1000 input fibers with an equal number of output fibers in a compact volume. This can be achieved with mirrors that can be densely packed together and that are rotatable by relatively large angles (xcx9c5xc2x0) in an arbitrary angular direction.
Recent developments in the field of microelectomechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches. MEMS-based mirrors and mirror arrays can be inexpensively designed and produced using conventional tools developed for the design and production of integrated circuits (IC""s). Such tools include computer aided design (CAD), photolithography, bulk and surface micromachining, wet and dry isotropic and anisotropic etching, and batch processing. In addition, deep reactive ion etching methods (DRIE) allow silicon devices to be produced having high aspect ratios (xcx9c20:1) that rival those that can be achieved using the prohibitively expensive lithography, electroplating and molding process (LIGA) which requires access to a synchrotron radiation source. (LIGA is an acronym for the German lithographie, galvanoformung und abformung).
A number of microelectromechanical mirror arrays have already been built using MEMS production processes and techniques. These arrays have designs that fall into approximately three design categories, each of which suffers from one or more limitations that make them sub-optimal for use in an all-optical cross connect switch.
The first and simplest design is illustrated by U.S. Pat. No. 5,960,132 to Lin. In this design, a reflective panel is hinged to a reference base and is electrostatically rotated about the hinge. Since the panel""s freedom of motion is limited to rotation about the hinge, light incident on the panel cannot be reflected in an arbitrary angular direction (dxcex8, dxcfx86) but only along an arc defined by a single angle, i.e., dxcex8 or dxcfx86. As a result, light incident from a source fiber cannot be directed to an arbitrary output fiber but only to those output fibers located along the defined arc. Consequently, Lin""s system requires large and costly system redundancies to connect a plurality of input fibers to a plurality of output fibers. These redundancies can be in either the number of output fibers or in the number of mirrors. In Lin, the redundancy is in the number of mirrors, where N2 mirrors are used to connect N input fibers to N output fibers. An optimal system would only require N mirrors to make the N input to N output possible fiber interconnections.
A more sophisticated design is illustrated in U.S. Pat. No. 6,044,705 to Neukermans et al which is hereby incorporated by reference. In Neukermans, a gimbal is mounted on a first hinge connected to a reference surface, while a mirror is mounted on a second hinge connected to the gimbal. The first and second hinges are orthogonal to each other and allow the mirror to be rotated in an arbitrarily angular direction (dxcex8, dxcfx86). The gimbal is electrostatically rotated about the first hinge by applying a potential between it and electrodes located on the reference surface. The mirror is electromagnetically rotated about the second hinge by injecting a current in a conductive coil wrapped around the mirror perimeter. The current flow through the coil generates a small magnetic moment which couples to a permanent magnetic field established across the plane of the mirror (e.g. with bar magnets), and causes the mirror to rotate. While Neukermans use of a gimbal thus allows the mirrored surface to rotate in an arbitrary angular direction, it also makes the system more mechanically and electrically complex than it needs to be. The mechanical complexity increases the sensitivity of the system to mechanical vibrations, while the electrical complexity increases the intricacy of the electrostatic and electromagnetic actuators. Both complexities increase the cost of producing the system. Additionally, Neukermans electromagnetic actuator coil occupies a large amount of the surface of the device, thus reducing the mirrored surface area and the mirror density.
A third mirror design is illustrated in U.S. Pat. No. 6,040,953 to Michalicek. In Michalicek, a mirror is mounted on a central post anchored to a locking pin joint that is carved into a reference surface. The post can be electrostatically actuated to freely rotate about the pin joint in an arbitrary direction. However, because the post is not mechanically attached to the pin joint with flexures, it can only be stably rotated in directions where the mirrored surface can be supported by a landing pad provided for that purpose. The mirror can therefore only be rotated and held in a fixed number of stable positions. In Michalicek""s preferred embodiment, the mirror can only be rotated to and held in two stable positions.
A freely rotatable, microelectromechanical mirror is disclosed. In one embodiment, the mirror can be used as a single switching element in an array of switching elements in an all-optical cross connect switch. The disclosed mirror includes a mirrored surface that can be mounted onto the top surface of a cylindrically shaped support post. The bottom surface of the support post can be mounted onto a freely movable, arbitrarily rotatable plate that is suspended in an actuation layer.
The actuation layer includes a freely movable plate suspended from and flexibly connected to a plurality of actuators that are themselves suspended from and flexibly connected to a support structure. In one embodiment, the support structure is a support frame held above a reference surface. In another embodiment the support structure is a plurality of support posts extending from the reference surface. The actuators can be actuated by any actuation means including electrostatic, electromagnetic, piezoelectric, and thermal actuation means. In one embodiment, the actuators are electrostatically actuated and the freely movable plate is suspended from and flexibly connected to three or more actuators that are distributed symmetrically about it. For example, in one preferred embodiment, the plate is suspended from and flexibly connected to four electrostatic actuators distributed about the plate at 90 degree intervals.
Each actuator is connected to the support structure by a plurality of actuator flexures that define a direction of motion in which the actuator can be actuated or moved. The direction of motion can be a linear direction of motion in which the actuator is translated, or an angular direction of motion in which the actuator is rotated. In a preferred embodiment, each actuator is connected to a support frame by a pair of torsional flexures that define an axis about which the actuator can be rotated. In this embodiment the actuator flexures functionally divide each actuator along its rotational axis into two ends that are respectively distal and proximal to the one or more plate flexures that connect the actuator to the freely movable plate.
Each of the plurality of actuators are connected to the freely movable plate by one or more plate flexures. In a preferred embodiment, each actuator is connected to the freely movable plate by two orthogonally oriented plate flexures. The first plate flexure connects the actuator to the second plate flexure and is configured to absorb energy transferred to the actuator from other components of the mirror. It serves to thereby decouple the actuator""s motion from the motion of other mirror components. The second plate flexure is configured to extend when the actuator is actuated, and to pull the freely movable plate in the direction of the resulting restoring force. In a preferred embodiment, the second plate flexure is configured to pull the freely movable plate toward or away from a reference surface when the actuator is rotated toward or away from the freely movable about the two flexures that connect the actuator to the support frame.
The actuation layer can be held above a reference surface by a number of standoff posts. In a preferred embodiment, the standoff posts electrically isolate the actuation layer from the reference layer, and a bias voltage is applied to the actuation layer. For each actuator suspended in the actuation layer, actuation means are disposed to cause the movement of the actuator. In one embodiment, the actuation means are control electrodes configured to electrostatically move the actuators. In other embodiments the actuation means can be current loops and magnetic fields configured to electromagnetically move the actuators, or piezoelectric crystals configured to piezoelectrically move the actuators.
The actuation means are attached to addressing and switching circuitry allowing individual actuators to be selectively moved. Selective movement of an actuator causes the plate flexure connecting the actuator to the freely movable plate to extend and to pull the freely movable plate in the direction of the actuator""s motion. Selective movement of two or more actuators allows for the selective movement of the freely movable plate by producing a net restoring force or a net torque on the plate. In one embodiment, the actuators can be selectively moved to rotate the freely movable plate in an arbitrary direction without stressing it. In another embodiment, the actuators can be selectively moved to translate the freely movable plate toward or away from a reference surface without rotating it.
For example, in a preferred embodiment each electrostatic actuator is suspended from a support frame by a pair of flexures about which it is configured to rotate. Distal and proximal control electrodes lie directly beneath the respective distal and proximal ends of each actuator. The control electrodes are attached to addressing circuitry that allows control voltages to be selectively applied to one or more of them. When a control voltage is applied to a control electrode, an attractive electrostatic force develops between the control electrode and the actuator lying above it, and causes the actuator to rotate toward the control electrode. As the actuator rotates toward a proximal (or distal) control electrode, it pushes (or pulls) the plate flexure connecting the actuator to the freely movable plate toward (or away from) the reference surface. In response, the plate flexure extends and exerts a restoring force that respectively pulls (or pushes) the freely movable plate toward (or away from) the reference surface, and toward the actuator.
When selective control voltages are applied to control electrodes lying beneath two or more electrostatic actuators, the actuators can be rotated in such a way that the plate flexures attaching the actuators to the freely movable plate create a net torque on the plate but no net force. Thus, the plate can be rotated about an arbitrary axis of rotation, defined by the net torque, without being translated or stressed. For example, when similar control voltages are applied to the proximal control electrode beneath a first electrostatic actuator attached to the freely movable plate, and to the distal control electrode beneath a second electrostatic actuator attached to the opposite side of the freely movable plate, the plate flexures attaching the actuators to the plate create a net torque on the plate but no net force. The net torque causes the plate to rotate, stress free, toward the first electrostatic actuator.
This freely movable, arbitrarily rotatable plate has several advantages over prior art actuated plates. First, since no net force is applied to the freely movable plate, it can be rotated without stressing it. Similarly, since the support post and mirrored surface are rigidly attached to the freely movable plate, they can be rotated stress free along with the plate. In addition, the opposing restoring forces created by opposing actuators that are selectively rotated allows a greater critical force to be applied to each of the actuators. The increased critical force allows a greater percentage (up to 80%) of the gap between the actuators and their control electrodes to be utilized, allowing the actuators to be rotated to larger critical angles than are possible in prior art actuators. Alternatively, the increased critical force allows the size of the gap between actuators and electrodes to be reduced, thereby allowing the actuators to be controllably rotated with smaller control voltages. Finally, the freely movable plate can be rotated by an angle that is magnified with respect to the angle by which the electrostatic actuators are rotated. The magnification factor is determined by the ratio of the distance from the flexures attaching the electrostatic actuators to the center of the freely movable plate, and the distance from the edge of the freely movable plate to its center.
In general, to rotate the mirror in an arbitrary angular direction (dxcex8, dxcfx86), a minimum of three electrostatic actuators must be connected to the freely movable plate. While the three or more electrostatic actuators need not be symmetrically distributed around the plate, certain advantages are achieved when they are so distributed. In particular, symmetrically distributing the electrostatic actuators around the freely movable plate simplifies the control voltages that need to be supplied to the control electrodes to rotate the plate and attached mirror in an arbitrary angular direction.
The details of various embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description, drawings and claims.